Short answer
You can repair stone chips, scratches and hairline cracks in the gelcoat yourself with a gelcoat repair kit. Clean, sand out the damage, fill with gelcoat mixed with hardener and cover it while it cures. Then sand it smooth and polish. If the fibreglass underneath is damaged, moisture has got into the laminate, or the damage is below the waterline near a through-hull fitting, have a yard look at it.
Assess the damage
| Damage | DIY? |
|---|---|
| Surface scratches | Yes, polishing is often enough |
| Chips, edge damage | Yes, gelcoat filler |
| Star-shaped hairline cracks | Yes, but look for the cause (stress, fittings) |
| Visible glass fibre or soft substrate | Yard. The laminate needs repair |
| Grounding damage to keel or hull | Yard, see grounding |
Step by step
- Clean with acetone to remove wax and grease. If old wax is left, the repair won't bond.
- Open up the damage with a small burr or knife into a clean V shape. Bevel the edges.
- Mix gelcoat and hardener exactly as the kit instructions say. Tint with pigment to match the shade. Old gelcoat has often yellowed.
- Fill slightly proud of the surface. Gelcoat cures poorly in air, so cover it with plastic film or use gelcoat with wax.
- Wet sand in stages, from coarser to progressively finer paper, until the repair is flush.
- Polish with rubbing compound and then fine polish, and finish with wax.
Tips
- Don't work in direct sun or cold. How the gelcoat cures depends on the temperature, and the kit states the range that applies.
- Mix small amounts, since the pot life is short.
- Wear a respirator and gloves. Styrene and sanding dust are hazardous to health.
- Below the waterline, the repair often needs epoxy and new antifouling, see antifouling.
Tools and materials
| You need | For |
|---|---|
| Gelcoat repair kit | Gelcoat, hardener, pigment |
| Acetone and rags | Degreasing before and between steps |
| Small router or multi-tool | Opening up the damage |
| Filling knife and plastic film | Filling and covering |
| Wet sandpaper in several grits | Sanding the repair down flush |
| Sanding block | A flat surface without dips |
| Rubbing compound, polish and wax | Bringing back the shine |
| Respirator and nitrile gloves | Protection against fumes and dust |
Expect to fill deep damage in two rounds, since gelcoat shrinks a little as it cures. Sand with a block. With your fingertips you get a dip that shows in backlight. Try it first on a hidden area so you see how quickly the material cures.
Save some of the tinted gelcoat, without hardener, for the next repair.
When it's a boatyard job
Large areas, osmosis blisters and structural damage often require lifting out, drying and laminating. Search for boatyards in your area under Marinas & boatyards and ask for a quote.
Frequently asked questions
Can I use ordinary plastic filler?
Under the gelcoat it's fine. The surface layer has to be gelcoat or topcoat, otherwise the filler soaks up water and the repair shows.
Why does the repair turn sticky?
Gelcoat without wax won't cure when exposed to air. Cover it with plastic film or use gelcoat with a wax additive.
How do I match the colour?
Mix pigment in small test batches and let them cure. Wet gelcoat looks different from cured.